A. Principles of Mechanical Ventilation
In many clinical settings mechanical ventilators are used to facilitate the respiratory flow of gas into and out of the lungs of patients who are sick, injured or anesthetized.
In general, mechanical ventilators provide a repetitive cycling of ventilatory flow, each such repetitive cycle being separated into two phases--an inspiratory phase followed by an expiratory phase.
The inspiratory phase of the ventilator cycle is characterized by the movement of positive-pressure inspiratory flow of gas through the ventilator circuit and into the lungs of the patient. The expiratory phase of the ventilatory cycle is characterized by cessation of the positive pressure inspiratory flow long enough to allow lung deflation to occur. The exhaled gas is vented from the ventilator circuit, typically through an exhalation valve. In patient whose lungs and thoracic musculature exhibit normal compliance, the act of exhalation is usually permitted to occur spontaneously without mechanical assistance from the ventilator.
It is sometimes desirable to control the airway pressure during exhalation to maintain a predetermined amount of positive back pressure during all, or a portion of, the respiratory cycle. Such techniques are often utilized to treat impairments of lung capacity due to pulmonary atelectasis or other factors.
The mechanical ventilators of the prior art have been grouped under various classification schemes, based on various criteria. In general, mechanical ventilators may be grouped or classified according to the parameter(s) which are utilized for a) triggering, b) limiting and c) terminating (e.g., cycling) the inspiratory phase of the ventilator cycle.
"Triggering" is the action that initiates the inspiratory phase of the ventilator cycle. The initiation of the inspiratory phase may be triggered by the ventilator or the patient. The variables and/or parameters which are utilized to trigger the beginning of the inspiratory phase include: time (i.e., respiratory rate), the commencement of spontaneous inhalation by the patient and/or combinations thereof.
"Limiting" of the inspiratory phase refers to the manner in which the inspiratory gas flow is maintained within prescribed ranges to optimize the ventilation of the patient's lungs. The limiting variables and/or parameters are typically controlled by the ventilator, but may change as a result of patient effort and/or physiologic variables such as lung compliance and airway resistance. The variables and/or parameters which are utilized for limiting the inspiratory phase include flow rate, airway pressure and delivered volume.
"Terminating" or "cycling" of the inspiratory phase of the ventilator cycle refers to the point at which the inspiratory flow is stopped and the ventilator and/or patient are permitted to "cycle" into the expiratory phase. Depending on the ventilator control settings, the termination of the inspiratory phase may be brought about by the ventilator or the patient. The variables and/or parameters which are utilized to terminate the inspiratory phase include: time; peak airway pressure; and/or tidal volume (V.sub.t).